1. Field of the Invention
The invention concerns integrated circuits and, more precisely, a detector of variations in the supply voltage Vcc of an integrated circuit, this detector forming part of the integrated circuit itself.
2. Description of the Prior Art
In certain applications, and especially in applications requiring confidentiality or safety of storage of the pieces of information memorized in the integrated circuit or processed in this circuit, it has been realized that it is possible to obtain access by fraudulent means to this confidential information and, as the case may be, to modify it by acting on the supply voltage Vcc of the circuit. It is possible, for example, to conceive of a situation where the confidentiality protection circuits no longer work accurately beyond a certain value of supply voltage.
Applications of this type are increasingly numerous and are illustrated notably by the integrated circuits mounted in chip cards, chip keys etc. These circuits are frequently associated with applications where confidentiality and security are sought.
Some of these circuits have been provided with detectors of any abnormal rise in the voltage Vcc above its nominal value or above the range of nominal values permitted in normal functioning. When the detector sees that the voltage Vcc has exceeded the nominal value (generally 5 volts) by 20% for example, it sends a signal barring all or a part of the functioning of the integrated circuit.
The detectors used for this purpose are threshold detectors, the threshold of which is chosen as a function of the acceptable limit for Vcc. They generally carry out their function accurately.
However, it has been realized, according to the present invention, that there are cases where the function is no longer fulfilled properly: the detector does not react in the requisite way to the variations in supply voltage.